The protein structure-function and/or exon-intron organization of lactate dehydrogenase genes A (muscle), B (heart) and C (testis) from human, mouse, rat, and Xenopus have been elucidated. In humans, enzyme deficiencies and variants of either LDH-A or LDH-B have been correlated with different clinical symptoms. The molecular nature of genetic mutations in human LDH-A and LDH-B mutant genes were analyzed by polymerase chain reaction, single-strand conformational polymorphism, and nucleotide sequencing. Among a total of 33 mutant genes characterized, 17 kinds of mutations included 9 missense mutations, 2 non-sense mutations, 4 deletions, and 2 insertions. The deletions (1bp, 2bp, 3bp, and 20bp) and insertions (4bp and 8bp) resulted in frameshift reading and prematrue translational terminations. Three of 9 missense mutations at the conserved amino acid sequences caused enzyme deficiency, while the other six missense mutations located near the surface of the subunit resulted in electrophoretic variants. We have also developed simple methods to detect these types of genetic mutations in human LDH-A(M) and LDH-B(H) genes. In collaboration with Prof. L.J. Guillette, Jr. of Florida, we have cloned LDH-A and/or LDH-B cDNAs from lizard and alligator in order to investigate the effect on expression of these LDH genes by environmental pollutants. The developmental expression of mouse LDH-A, LDH-B and LDH-C genes during spermatogenesis and oogenesis, as well as the induction of LDH-A gene by cAMP and estrogen, have been elucidated. The molecular evolution and mechanism(s) of LDH gene regulation will be investigated.